JP3249375B2 - Optical recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention can be applied to an optical recording medium capable of storing information at a high density, in particular, to a two-layer type high-density optical disk for reproducing by irradiating a reproducing light from one side. It relates to an optical recording medium.

[0002]

2. Description of the Related Art In recent years, digital video disks (DVDs) capable of recording moving images such as movies on a compact disk having a diameter of 120 mm for two hours or more have been actively developed. Such a DVD has been proposed which has two signal layers in order to increase the recording capacity.

[0003] As such a two-layer optical disc, a two-sided reading system in which two resin substrates on which signal pits are formed are bonded with their signal surfaces facing each other and a reflection layer is formed on each signal surface side. There has been proposed a two-layer optical disc of a single-sided reading type in which a layered optical disc and one reflective layer are formed as a semi-transparent layer, and reproduction light is irradiated from only one side and reproduced.

In a multimedia CD (MMCD), a first reflective film made of a translucent film is formed on a transparent resin substrate, and a second reflective film made of a metal film is formed on the first reflective film via a transparent film. A two-layer optical disc having a reflective film formed thereon is known.

[0005]

In the above-described DVD double-layer optical disk and MMCD of the one-side reading system, a transparent resin layer is provided between two signal layers.
The signal layer far from the reproduction light incident side is irradiated with the reproduction light having passed through the transparent resin layer. The present inventors have found that when the second signal layer is reproduced by the reproduction light having passed through the transparent resin layer, the quality of the reproduction signal is reduced. That is, it has been found that the transparent resin layer has birefringence, and therefore, when reproduced by the reproduction light passing through the transparent resin layer, the quality of the reproduced signal is deteriorated due to the influence of the birefringence.

An object of the present invention is to provide an optical recording medium in which a plurality of signal layers are provided via a transparent resin layer, such as a two-layer type optical disk for reproducing by irradiating the reproducing light from one side, and reproducing light. An object of the present invention is to provide an optical recording medium capable of reducing deterioration of a reproduction signal caused by passing through a transparent resin layer.

[0007]

The optical recording medium of the present invention comprises:
An optical recording medium on which information stored in a signal layer in which information is stored is reproduced by irradiating the signal layer with the reproduction light, and a first signal is provided as a signal layer in order from a side where the reproduction light is incident. A first resin layer and a second signal layer, and a transparent resin layer containing a transparent resin material as a main component and provided between the first signal layer and the second signal layer. It is characterized in that a material having a refractive property is present between the first signal layer and the second signal layer.

In one preferred embodiment according to the present invention, a material having a birefringence opposite to that of the transparent resin material is mixed in the transparent resin layer. Such a material is preferably mixed with the transparent resin material forming the transparent resin layer, and the mixed material is used to form the transparent resin layer by spin coating or the like.

In the present invention, “transparent” in the “transparent resin layer” and the “transparent resin material” means that it is transparent to reproduction light. Further, the material having the opposite birefringence to the transparent resin material used in the present invention is also preferably transparent to the reproduction light.

The signal layer according to the present invention indicates an area where information is stored in an optical recording medium.
When a reflective surface having recording pits and having irregularities, such as a compact disk (CD), becomes an area for storing information, the reflecting surface having irregularities with the recording pits formed becomes a signal layer.

In the present invention, examples of the material having the opposite birefringence to the transparent resin material include a monomer having the opposite birefringence to the transparent resin material or a polymer of these monomers. For example, when the transparent resin material is a polymer having a positive birefringence, a monomer having a negative birefringence and / or a polymer thereof can be used. As described above, in a preferred embodiment according to the present invention, such a monomer having negative birefringence and / or a polymer thereof is mixed into a transparent resin material to form a transparent resin layer.

In the present invention, the amount of the material having the opposite birefringence is not particularly limited. For example, as will be described later, the material is actually mixed in a transparent resin material to measure the birefringence. According to the method described above, the amount that most reduces birefringence can be determined and used.

In a conventional optical recording medium such as a two-layer optical disk, the reason why the transparent resin layer serving as an intermediate layer between the first signal layer and the second signal layer has birefringence is as follows. That is, polymers generally have large birefringence. For example, when molded in a state in which stress such as spin coating or stretching is applied, birefringence occurs according to the direction of the stress. Therefore, it is considered that the transparent resin layer is formed in a state in which stress is applied, thereby causing birefringence.
When light is incident on such a birefringent polymer, the phase of the polarized light component parallel to the direction of the stress is advanced or delayed with respect to the perpendicularly polarized light component. Such leading and lagging phases have a birefringent relationship of opposite signs to each other and depend on the type of polymer.

In the present invention, by providing between the first signal layer and the second signal layer, a material having a birefringence opposite to that of the transparent resin material as the main component of the transparent resin layer is provided. The birefringence of the transparent resin layer is reduced. Further, as described above, in a preferred embodiment, a material having the opposite sign of birefringence is mixed in the transparent resin layer.

FIG. 2 is a view for explaining the principle that the birefringence of the transparent resin layer can be reduced by the presence of a material having a birefringence opposite to that of the transparent resin material in the present invention. It is. FIG. 2A shows a state similar to that of a conventional transparent resin layer containing only a transparent resin material. In FIG. 2A, the direction indicated by the arrow is
The direction of the stress applied to the transparent resin material 9 when forming the transparent resin layer is shown, and the transparent resin material 9 is oriented in this direction. Due to such orientation of the transparent resin material 9,
Birefringence occurs.

FIG. 2B shows a state in which a material having the opposite sign of birefringence is contained according to the present invention. FIG.
As shown in (b), it can be seen that the material 10 having the opposite sign of birefringence is oriented in the direction perpendicular to the direction of orientation of the transparent resin material 9. Thus, for the transparent resin material 9,
A material 10 having a birefringence opposite to that of the transparent resin material 9
, The birefringence is offset as a whole, and the birefringence is reduced. Therefore, the influence of the birefringence on the reproduction light passing through the transparent resin layer is reduced, and the quality of the reproduction signal can be improved.

[0017]

FIG. 1 is a sectional view showing an optical disk according to an embodiment of the present invention. Referring to FIG. 1, the optical disc of the present embodiment is formed by bonding a transparent substrate 1 and a transparent substrate 2 with a transparent resin layer 5 made of an ultraviolet curable resin as a transparent resin material. As the transparent substrates 1 and 2, a substrate material such as PC (polycarbonate) or PMMA (polymethyl methacrylate) is used.

On the inner surface of the substrate 1, a first signal layer 3 is formed. The first signal layer 3 is formed by forming a pit on the inner surface of the substrate 1 and forming a semi-transparent film made of a dielectric film such as ZnS on the pit.

On the inner surface of the substrate 2, a second signal layer 4 is formed. The second signal layer 4 is formed by forming recording pits on the inner side surface of the substrate 2 and forming a reflective film made of a metal film such as an Al film thereon.

The reflection film made of the translucent film of the first signal layer and the reflection film of the second signal layer can be formed by, for example, a sputtering method. As shown in FIG. 1, the thickness of each of the substrate 1 and the substrate 2 is about 0.6 mm.
The transparent resin layer 5 is formed to have a thickness of about 40 μm.

To reproduce the first signal layer 3, for example, a laser light (reproduction light) 6 of a red semiconductor laser is applied to an objective lens 8.
Then, light can be condensed on the first signal layer 3 and the reflected light can be detected to reproduce the signal. The first signal layer 3 has a reflectance of, for example, about 30%.

When reproducing the second signal layer 4, for example, a laser beam (reproducing light) 7 of a red semiconductor laser is converged on the second signal layer 4 by squeezing it with an objective lens 8. Then, by detecting the reproduction light reflected by the second signal layer 4, the information stored in the second signal layer 4 can be reproduced.

At the time of reproducing the second signal layer 4, the laser beam 7 passes through the transparent resin layer 5 and reaches the second signal layer 4. Therefore, when the transparent resin layer 5 is made of only a transparent resin material and has birefringence as in the conventional case, the reproduction light reflected by the second signal layer 4 is affected by the birefringence, and The quality deteriorates. In the present embodiment, in order to reduce the birefringence of the transparent resin layer 5, a material having the opposite birefringence to the transparent resin material is added and mixed in the transparent resin layer 5.

FIG. 3 is a schematic diagram for explaining a material having the opposite birefringence and a method of evaluating the birefringence for determining the amount of the material added. The birefringence evaluation method described here is an example, and the birefringence may be evaluated by other evaluation methods to determine the material to be added and the amount thereof. Referring to FIG.
Is used as a light source 11, and polarizers 13 in a crossed Nicols arrangement are provided on both sides of a sample 14, the laser light from the light source 11 is shaped by a collimator lens 12, and the sample 14 passes through one of the polarizers 13. To the photodetector 1 through the other of the polarizers 13
5 to detect. If the sample 14 has birefringence, the amount of transmitted light increases according to the birefringence,
By detecting this with the photodetector 15, the birefringence of the sample 14 can be evaluated.

As a sample 14, an ultraviolet curable resin (trade name "UVS HC-329", MOROHOSHI PRINTING INK C) conventionally used as a transparent resin material has been used.
O., LTD), and mixed with polystyrene, polymethyl methacrylate (PMMA) and trans-stilbene at various concentrations. After that, the birefringence was evaluated using an apparatus shown in FIG. As a result, in the case of polystyrene and poly (methyl methacrylate), the birefringence increased when the content concentration in the ultraviolet curable resin was increased, whereas in the case of trans-stilbene, when mixed with the ultraviolet curable resin, 3 wt% It was confirmed that the birefringence decreased monotonically up to the concentration, and the birefringence completely disappeared at 3% by weight, and the birefringence increased again when the concentration was further increased thereafter. This is because polystyrene and poly (methyl methacrylate) have the same sign of birefringence as UV-curable resin, while trans-stilbene has the opposite sign of birefringence with UV-curable resin, Is 3% by weight, it is considered that the birefringence was offset.
Therefore, in the embodiment shown in FIG. 1, a material obtained by mixing trans-stilbene with a 3% by weight ultraviolet curable resin is used as the transparent resin layer 5.

In the embodiment shown in FIG. 1, the recording pits of the first signal layer 3 and the second signal layer 4 are EFM signals of the shortest pit 0.4 μm, and the track pitch is 0.8.
It was 2 μm. The signal layers were formed such that the first signal layer 3 and the second signal layer 4 had spirals opposite to each other. When the second signal layer 4 was reproduced using a red semiconductor laser having a wavelength of 650 nm as a reproduction light source and a pickup having a numerical aperture of an objective lens of 0.6, the jitter value was 4.5%. As a comparison, an optical disc was produced in the same manner as in the embodiment shown in FIG. 1 except that a transparent resin layer was formed using an ultraviolet curable resin in which trans-stilbene was not mixed. Upon reproduction, the jitter value was 8.5%. Therefore, by providing a trans-stilbene having a birefringence opposite to that of the transparent resin material between the first signal layer and the second signal layer according to the present invention, the jitter value is greatly improved, and Quality could be greatly improved.

FIG. 4 is a perspective view showing an optical disc of another embodiment according to the present invention. The optical disk shown in FIG.
This is a disk structure generally called MMCD. Referring to FIG. 4, a translucent film formed of a dielectric film such as ZnS is formed on a transparent substrate 21 formed of PC or the like. Reflective film (1st
Signal layer) 22. On the first reflective film 22,
A transparent resin layer 24 made of an ultraviolet curable resin or the like is provided. On the transparent resin layer 24, a metal reflection film made of an Al film or the like is formed. This metal reflective film becomes the second reflective film (second signal layer) 23. Recording pits 25 are formed on the first reflective film 22 and the second reflective film 23.
Is formed and information is stored. As shown in FIG. 4, in the present embodiment, the thickness of the transparent substrate 21 is about 1.
2 mm, and the thickness of the transparent resin layer 24 is about 40 μm.

In the optical disc of the present embodiment, when reproducing the second layer of the reflection film 23, the reproduction light passes through the transparent substrate 21 and the first layer of the reflection film 22 and further passes through the transparent resin layer 24.
And is applied to the second reflective film 23. Therefore,
When the transparent resin layer 24 has birefringence, it is affected by this birefringence. In the present embodiment, as in the embodiment shown in FIG. 1, a transparent resin layer 24 is formed by using a mixture of trans-stilbene and 3% by weight of an ultraviolet curable resin. Therefore, the birefringence of the transparent resin layer 24 is greatly reduced, and the quality of the reproduced signal from the second reflective film 23 can be greatly improved.

In the above embodiment, a description has been given by taking a read-only type two-layer type optical disk as an example. However, the present invention is not limited to this. For example, the first signal layer and the second signal layer The invention can also be applied to an optical disk of which at least one is a rewritable type. For example, the present invention can be applied to a two-layer type optical disc having a rewritable signal layer and a read-only signal layer by forming a phase change thin film on one of the first recording layer and the second recording layer.

In the above embodiment, trans-stilbene was used as the material having the opposite birefringence. However, the present invention is not limited to this, and the transparent resin material which is the main component of the transparent resin layer is used. Various low-molecular compounds, monomers, and polymers can be used as long as the material can reduce birefringence by mixing with the above.

[0031]

According to the optical recording medium of the present invention, a material having a birefringence opposite to that of the transparent resin material is present between the first signal layer and the second signal layer. Birefringence is reduced. Therefore, it is possible to reduce the influence of the birefringence due to the transparent resin layer when reproducing by irradiating the signal layer with reproduction light, and it is possible to obtain a high-quality reproduction signal.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an optical disc according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing an orientation state of a transparent resin material in a transparent resin layer and an orientation state of a material having reverse birefringence.

FIG. 3 is a schematic diagram for explaining a method of evaluating birefringence used in an embodiment according to the present invention.

FIG. 4 is a perspective view showing an optical disc of another embodiment according to the present invention.

[Explanation of symbols]

 1, 2 ... substrate 3 ... first signal layer 4 ... second signal layer 5 ... transparent resin layer 6, 7 ... laser beam 8 ... lens 21 ... transparent substrate 22 ... first layer reflection film 23 ... second layer Reflective film 24 ... Transparent resin layer 25 ... Pit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G11B 7/24

Claims (2)

(57) [Claims] 1. An optical recording medium on which information stored in a signal layer is reproduced by irradiating the signal layer with the information with reproduction light, wherein the signal layer has a side on which the reproduction light is incident. A first signal layer and a second signal layer provided in this order, and a transparent resin layer containing a transparent resin material as a main component and provided between the first signal layer and the second signal layer. An optical recording medium comprising: a material having a birefringence opposite to that of the transparent resin material, between the first signal layer and the second signal layer. 2. The optical recording medium according to claim 1, wherein the material having the opposite birefringence is mixed in the transparent resin layer.